The invention relates to a brazing product, such as a brazing sheet product, having an aluminium layer being made of an aluminium alloy comprising silicon in an amount in the range of 2 to 18% by weight, and a further layer comprising nickel on the outer surface of the AlSi-alloy layer such that taken together the aluminium layer and all layers exterior thereto form a filler metal for a brazing operation. The invention also relates to a method of manufacturing a brazed assembly using the brazing product, and to a brazed assembly comprising at least one component made of the brazing product.
Aluminium and aluminium alloys can be joined by a wide variety of brazing and soldering processes. Brazing, by definition, employs filler metal having a liquidus above 450xc2x0 C. and below the solidus of the base metal. Brazing is distinguished from soldering by the melting point of the filler metal: solders melt below 450xc2x0 C. Soldering processes are not within the field of the present invention.
Brazing products, and in particular brazing sheet products, find wide applications in heat exchangers and other similar equipment. Conventional brazing sheet products having a core sheet, typically an aluminium alloy of the Aluminium Association (AA)3xxx-series, having on at least one surface of the core sheet clad an aluminium clad layer, the aluminium clad layer being made of an AA4xxx-series alloy comprising silicon in an amount in the range of 2 to 18% by weight, and preferably in the range of 7 to 14% by weight. The aluminium clad layer may be coupled to the core alloy in various ways known in the art, for example by means of roll bonding, cladding, explosive cladding, thermal spray-forming or semi-continuous or continuous casting processes.
Controlled Atmosphere Brazing (xe2x80x9cCABxe2x80x9d) and Vacuum Brazing (xe2x80x9cVBxe2x80x9d) are the two main processes used for industrial scale aluminium brazing. Industrial vacuum brazing has been used since the 1950""s, while CAB became popular in the early 1980""s after the introduction of the NOCOLOK (trade mark) brazing flux. Vacuum brazing is an essentially discontinuous process and puts high demands on material cleanliness. The disruption of the oxide layer present is mainly caused by the evaporation of magnesium from the clad alloy. There is always more magnesium present in the furnace than necessary. The excess magnesium condenses on the cold spots in the furnace and has to be removed frequently. The capital investment for suitable equipment is relatively high.
CAB requires an additional process step prior to brazing as compared to VB, since a brazing flux has to be applied prior to brazing. CAB is essentially a continuous process in which, if the proper brazing flux is being used, high volumes of brazed assemblies can be manufactured. The brazing flux dissolves the oxide layer at brazing temperature allowing the clad alloy to flow properly. When the NOCOLOK flux is used the surface needs to be cleaned thoroughly prior to flux application. To obtain good brazing results the brazing flux has to be applied on the total surface of the brazed assembly. This can cause difficulties with certain types of assemblies because of their design. For example, because evaporator type heat exchangers have a large internal surface, problems can arise because of poor access to the interior. For good brazing results the flux has to adhere to the aluminium surface before brazing. Unfortunately the brazing flux after drying can easily fall off due to small mechanical vibrations. During the brazing cycle, corrosive fumes such as HF are generated. This puts a high demand on the corrosion resistance of the materials applied for the furnace.
Ideally, a material should be available that can be used for CAB but does not have the requirements and defects of the known brazing flux application. Such a material can be supplied to a manufacturer of brazed assemblies and is ready to use directly after forming of the assembly parts. No additional brazing fluxing operations have to be carried out. Presently, only one process for flux-less brazing is used on an industrial scale. The material for this process can be for example standard brazing sheet made from an AA3xxx-series core alloy clad on both sides with a cladding of an AA4xxx-series alloy. Before the brazing sheet can be used the surface has to be modified in such a way that the naturally occurring oxide layer does not interfere during the brazing cycle. The method of achieving good brazing is to deposit a specific amount of nickel on the surface of the clad alloy. If properly applied, the nickel reacts, presumably exothermically, with the underlying aluminium. The nickel can be applied by using a shim of nickel between the two parts to be joined or can be deposited by electroplating. When electroplating is used the adherence of the nickel should be sufficient to withstand typical shaping operations being used in for example heat exchanger manufacture.
Processes for nickel-plating in an alkaline solution of aluminium brazing sheet are known from each of U.S. Pat. No. 3,970,237, 4,028,200, 4,164,454, and SAE-paper no. 880446 by B. E. Cheadle and K. F. Dockus. According to these documents, nickel or cobalt, or combinations thereof, are most preferably deposited in combination with lead. The lead addition is used to improve the wetability of the aluminium clad alloy during the brazing cycle. An important characteristic of these plating processes is that the nickel is preferentially deposited on the silicon particles of the aluminium clad alloy. To obtain sufficient nickel for brazing, the surface of the aluminium clad alloy should contain a relatively large number of silicon particles to act as nuclei for the nickel deposition. It is believed that to obtain sufficient nucleation sites a part of the aluminium in which the silicon particles are embedded should be removed by before pickling chemical and/or mechanical pretreatment. This is believed a necessary condition to obtain sufficient nickel coverage to serve as nuclei for the plating action of the brazing or clad alloy. On a microscopic scale the surface of the Si-containing cladding of the brazing sheet is covered with nickel globules. However, the use of lead for the production of a suitable nickel and/or cobalt layer on brazing sheet has several disadvantages. The use of lead for manufacturing products, such as automotive products, is undesirable and it is envisaged that in the very near future there might possibly even be a ban on lead comprising products or products manufactured via one or more intermediate processing steps comprising lead or lead-based components.
In the international PCT patent application no. WO-00/71784, incorporated herein by reference, J. N. Mooij et al. disclose a brazing sheet product and a method of its manufacture. In this brazing sheet product there is provided a bonding layer comprising zinc or tin between the AlSi-alloy clad layer and the nickel layer in order to improve the bonding of the nickel layer. The addition of lead to the nickel layer has been replaced by the addition of bismuth while maintaining the excellent brazeability characteristics of the brazing sheet product.
A drawback of the known brazing sheet products having a layer comprising nickel is the limited corrosion life of brazed products in a SWAAT-test in accordance with ASTM G-85. Corrosion lifetimes without perforations are typically in the range of 5 to 7 days. For several applications of the known nickel-plated brazing sheet in brazed products such a relatively short corrosion lifetime is not detrimental. However, a good corrosion resistance is an important property for brazing products used in heat exchangers, such as radiators, condensers and oil coolers. These heat exchangers are exposed to a severe external corrosive attack by, e.g., de-icing road salt. Long-life alloys are considered herein as those, which in the SWAAT-test without perforations according to ASTM G-85 exceed 10-12 days (see e.g. K. Scholin et al., VTMS 1993, SAE P-263).
It is an object of the present invention to provide a Ni-plated brazing product for use in a brazing operation, ideally a CAB brazing operation, and wherein the brazing product has an improved post-braze corrosion resistance when measured in a SWAAT-test in accordance with ASTM G-85 (which for purposes of this disclosure is the version of year 1990).
It is another object of the present invention to provide a brazing sheet product having a core sheet made of an aluminium alloy coupled on at least one surface of the core sheet to an aluminium clad layer, the aluminium clad layer being made of an aluminium alloy comprising silicon in an amount in the range of 2 to 18% by weight, and a further layer comprising nickel on the outer surface of the aluminium clad layer such that taken together the aluminium clad layer and all layers exterior thereto form a filler metal for a brazing operation, and wherein the brazing sheet product has an improved corrosion resistance when measured in a SWAAT-test in accordance with ASTM G-85.
It is another object of the present invention to provide a brazing product, ideally a brazing sheet product, having had a corrosion life of 12 days or more in a SWAAT-test without perforations in accordance with ASTM G-85.
It is another object of the present invention to provide a method of manufacturing brazed assemblies using the brazing product.
A brazing product, e.g. a brazing sheet product, having an aluminium layer made of an aluminium alloy comprising silicon in an amount in the range of 2 to 18 weight %, and a layer comprising nickel on the outer surface of the aluminium layer such that taken together the aluminium layer and all layers exterior thereto form the filler metal for a brazing operation. The filler metal has a composition containing at least one element: (i) with a smaller exchange current density for the Hydrogen Evolution Reaction (xe2x80x9cHERxe2x80x9d) than nickel and/or (ii) such that the electro-chemical potential difference between particles of Ni-aluminide(s) of the filler and the aluminium alloy matrix of the filler is reduced. The filler metal""s mol-ratio of Ni to such element(s) is in the range of 10:(0.3-30). The invention also relates to a method of manufacturing a brazed assembly using the brazing product, and to a brazed assembly comprising at least one component made of the brazing product.